The present invention relates to nuclear fuel assemblies and more particularly to hold-down devices for nuclear fuel assemblies. More particularly still the invention, in its preferred embodiment, provides an improved fuel assembly hold-down device having additional utility as the lifting surface during handling of the fuel assembly.
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements grouped in and supported by frameworks referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower alignment and support grids. These grids, often referred to as support or alignment plates, are themselves directly or indirectly attached to a support barrel which surrounds the entire core and extends beyond the ends thereof. In the most common configuration the axis of the core support barrel extends vertically and the various fuel assemblies are also arranged vertically and receive support by resting at their lower ends upon the lower support plate.
Local temperatures at various locations within the core may vary greatly and accordingly the expansion experienced by the various materials and elements in the core region may vary from location to location. Further because the materials used in the core region are not all the same, they exhibit different thermal growth characteristics. The thermal expansion of the various members in the core region in the axial or vertical direction may be quite significant, particularly at the temperatures found within a nuclear reactor and because the length of some of the members involved may be 12-15feet or more. For these reasons, the fuel assemblies are not usually attached to the upper and lower alignment plates but rather are supported in a manner which permits some relative motion therebetween. This axial thermal expansion differential between the fuel assemblies and the core support barrel is generally accommodated by insuring that the axial spacing between the upper and lower core support and alignment plates is somewhat greater than the axial length of the fuel assemblies for the entire range of thermal conditions in the core region.
In order to facilitate handling and installation of the fuel assemblies, they are generally not secured to the lower core support plate and rely upon axially movable alignment posts extending downwardly through guide holes in the support plate for lateral alignment. In most reactors a fluid coolant, such as water, is directed upwardly through apertures in the lower core support plate and along the fuel rods in the various fuel assemblies to receive the thermal energy therefrom. The physical configuration of the various fuel assemblies is such that the coolant may experience a significant pressure drop in passing upwardly through the core region. This pressure drop necessarily produces a lifting force on the fuel assemblies. In some instances, the weight of the fuel assembly is sufficient to overcome the upward hydraulic lifting forces under all operating conditions. However, this is often not the case, particularly when the coolant density is high as at reactor startup and additionally because of increasing coolant flow rates. When the hydraulic forces in the upward direction on a particular fuel assembly are greater than the weight of that fuel assembly, the net resultant forces on the fuel assembly will be in the upward direction and will cause the assembly to move upward into contact with the upper core alignment plate. This motion of a fuel assembly, if uncontrolled, may result in damage to the fuel assembly and its fuel rods or to the upper alignment plate and must, therefore, be avoided. In order to prevent hydraulic lifting of the fuel assemblies various hold-down devices have been employed.
For the most part the vertically extending structural members of a fuel assembly and the core support barrel have been of the same material, stainless steel. Because they have been of the same material, the axial thermal expansion differential between them has been greatly limited and only small variations in the spacing between the upper end of the fuel assembly and the upper core alignment plate have existed. Leaf springs acting between the upper core alignment plate and the upper end of a fuel assembly have generally been sufficient to overcome any lifting of the fuel assemblies.
More recently, however, the vertically extending structural members of a fuel assembly have been fabricated of Zircaloy. This is particularly the case when the vertically extending structural members of a fuel assembly are the guide tubes into which control rod fingers are inserted. Because the materials used in the vertically extending support structure of the fuel assemblies are different than that used in the core support barrel, the opportunity for a significant thermal expansion differential is created. Increasing temperatures in the core region and increasing length of fuel assemblies serve to further aggrevate the thermal expansion differential problem. As an example, in present reactors having a stainless steel core support barrel and fuel assemblies supported by Zircaloy guide tubes, the gap between the fuel assembly and the upper core alignment plate may vary 5/8 of an inch or more due to the linear expansion differential. The hold-down means employed must be capable of providing an adequate hold-down force to the fuel assembly over the entire possible gap range. The leaf spring, however, is inherently a low deflection device and for the spring size limitations dictated by the reactor core environment, is generally incapable of providing the necessary hold-down forces over the entire range of gap distances which might be encountered.